787 Battery Safety 'Must Be Reconsidered': NTSB

CNBC's Phil LeBeau reports the NTSB has not yet determined what caused the short circuiting in one of the cells in the 787's lithium ion battery.

Three weeks after the Federal Aviation Administration grounded Boeing 787 Dreamliners, the head of the National Transportation Safety Board said assumptions about the planes' lithium-ion batteries should be reevaluated.

"Assumptions used to certify the battery must be reconsidered," said Deborah Hersman, chairman of the NTSB. "Boeing assessed that the likelihood of a smoke emission event from a 787 battery would occur less than once in every 10 million flight hours. The 787 fleet has accumulated less than 100,000 flight hours, yet there have now been 2 battery events resulting in smoke less than two weeks apart, on two different aircraft."

NTSB investigators have determined the source of the battery fire in a Japan Airlines Dreamliner on Jan. 8 in Boston. Short circuiting in one of the cells in a lithium-ion battery sparked the 787 fire. The short circuit occurred in cell number six of the eight cells that make up the battery.

Saul Loeb | AFP | Getty Images

The damaged battery case from a fire aboard a Japan Airlines (JAL) Boeing 787 Dreamliner airplane at Logan International Airport in Boston is displayed inside an investigation lab at National Transportation Safety Board (NTSB) Headquarters in Washington, DC.

"We have not yet identified what the cause of the short circuit is. We are looking at manufacturing, design, and cell charging," Hersman said.

While the NTSB was updating its investigation into the 787, a Dreamliner was in flight from Ft. Worth, Texas to Everett, Washington. The plane was being painted in Texas and Boeing wanted to bring it back to the Seattle area so it will be in place for any modifications that will be needed when the FAA lifts the grounding of Dreamliners.